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Normal RPC operation is to supply a receive port to mach_msg() with MACH_MSG_TYPE_MAKE_SEND_ONCE. In that case, rpctrace sees the send once port for the first time when the RPC message is sent.

This code first extracts a send once right from the receive port, then uses it during the upcoming RPC call. This causes rpctrace to see the port an extra time when it comes back in the reply to mach_port_extract_right(), and it gets wrapped. rpctrace then sees it again during the actual RPC call, and double wraps it, which ultimately triggers a failed assert when it tries to match a request to the second reply.